Absolutely Small - Michael D. Fayer [109]
CHEMICALLY MODIFIED FATS
Fats are chemically modified for several reasons. Unsaturated fats are fats with double bonds. Double bonds are chemically very reactive. The double bonds in unsaturated fats can react with oxygen. When this occurs to a significant extent, the smell and taste of the oil becomes unpleasant; we say the oil has become rancid. The rates of reactions of oxygen with unsaturated fats are exacerbated by light.
Polyunsaturated fats have more double bonds. With more double bonds available to react with oxygen, the polyunsaturated oil will become rancid more readily. Oils containing unsaturated fats should be refrigerated. Lowering the temperature slows the chemical reactions that cause rancidity. If such oils are not refrigerated, it is better to keep them in a cool, dark place. Some oils come in dark bottles. The dark bottle may improve their shelf life in a store where they are exposed to light. Oils that are composed of almost all saturated fats will keep for a long time without refrigeration. The result is that many oils are chemically processed to reduce or eliminate the double bonds. Such processing also changes the physical properties of the oils, raising the temperatures at which they melt and boil. These changes can be useful in cooking, in preparing baked goods, and in other processed food applications.
PARTIALLY HYDROGENATED AND HYDROGENATED FATS
The oils that are chemically modified to reduce or eliminate double bonds are said to be partially hydrogenated or hydrogenated. A carbon atom makes four bonds. In saturated fats (no double bonds), each carbon is bonded to two other carbons and to two hydrogens, except for the carbons at the ends of a molecule. A double bond eliminates two hydrogen atoms. This can be seen by comparing stearic acid (Figure 16.1) and oleic acid (Figure 16.2). Formation of the double bond in oleic acid between carbons 9 and 10 uses up one bond on each of the two carbon atoms that in stearic acid form bonds to hydrogen atoms. Therefore, the process of eliminating double bonds from a fat increases the number of hydrogen atoms, and we say the oil has been hydrogenated.
HYDROGENATION OF FATS
Double bonds are very stable, and it is difficult to break a double bond. The hydrogenation process turns a carbon-carbon double bond into a carbon-carbon single bond with a hydrogen added to each carbon. The process requires high temperature, a metallic catalyst, and hydrogen. A catalyst is a material that makes a chemical reaction occur faster, but the catalyst is not used up in the process. Qualitatively, the process of hydrogenation works as follows. One of the carbon atoms of a double bond binds to the metal, essentially eliminating the double bond. The binding leaves the other carbon atom with an unpaired electron. That carbon atom is now single bonded to two other carbons and one hydrogen. As we know, a carbon atom wants to make four bonds to have enough electrons to obtain the neon closed shell configuration. The carbon grabs a hydrogen atom. The other carbon atom breaks its bond to the metal catalyst, and it grabs another hydrogen atom. The carbon-carbon double bond has been converted to a single bond and two hydrogen atoms have been added to the fat.
For a polyunsaturated fat, this process can be repeated at all of the double bonds or only at some of them. If it occurs at all of the double bonds, the polyunsaturated fat has been converted to a saturated fat; we say the fat has been hydrogenated. If hydrogenation occurs at some but not all of the double bonds, we say the fat has been partially hydrogenated. The resulting fat may be monounsaturated or still polyunsaturated but with fewer double bonds. The degree of hydrogenation is controlled to produce a resulting fat with the desired properties, particularly whether it is a solid or liquid at room temperature, the